1. Field of the Invention
The present invention relates to a lighting device that is suitable for use in various light sources, e.g., a backlight light source for a liquid-crystal television.
2. Description of the Related Art
Recently, a lighting device comprising a light emitting diode (=LED) device has been adopted instead of a cold-cathode fluorescent tube containing a mercury gas therein, from the viewpoints of eliminating the harmful mercury gas on the environment (or human body), preventing component parts from deteriorating by ultraviolet radiation, decreasing the electric power consumption cost due to the use of an inverter circuit, and obtaining a good color reproducibility.
Generally, in this kind of lighting device, to obtain white light, red, green and blue light-emitting diode devices, or blue-green and yellow light-emitting diode devices are used.
However, since this kind of lighting device uses semiconductor light-emitting diode devices comprising different semiconductor materials respectively, the LED devices each require a different driving electric power so that its electric current needs to be adjusted by ensuring an electric power source for each of the devices. Further, since the light-emitting diode devices each are a semiconductor light-emitting diode device, temperature characteristics and deterioration with time of the devices are different respectively so that color tones thereof may change separately. Further, unless lights emitted from the light-emitting diode devices are evenly mixed, unevenness of color may occur.
In order to avoid the disadvantages described above, a lighting device is disclosed which can obtain white light by using a single light-emitting diode device.
This kind of a known lighting device comprises a housing with an opening facing an illuminated object and electrodes, a light-emitting diode device disposed in the housing, and a mold member containing a phosphor for sealing the light-emitting diode device (e.g., JP-A-10-242513).
JP-A-10-242513 also discloses a lighting device which comprises two leads a part of which is exposed to the outside, a light-emitting diode device mounted on one lead (=mount lead) of the two leads, a first mold member containing the phosphor for sealing the light-emitting diode device, and a second mold member with an optical shape surface for sealing the light-emitting diode device and the lead (a part thereof).
Further, another conventional lighting device is proposed which comprises two leads a part of which is exposed to the outside, a light-emitting diode device mounted on one lead (=mount lead) of the two leads, a mold member with an optical shape surface for sealing the light-emitting diode device, and a phosphor member laminated on the optical shape surface of the mold member.
In the three lighting devices described above, the light-emitting diode device to emit blue light, and the phosphor to emit yellow light by being excited by the blue light are used such that that white light is radiated, toward the illuminated object, as a mixture of the blue light emitted from the light-emitting diode device and the yellow light emitted from the phosphor.
However, the lighting devices shown in JP-A-10-242513, i.e., the two lighting devices comprising the mold member containing the phosphor and the other lighting device comprising the phosphor member laminated on the optical shape surface of the mold member, have the disadvantage that the phosphor and the mold member deteriorate by heat generated from the light-emitting diode device during the operation so that the former (i.e. , the lighting devices shown in JP-A-10-242513) will be subjected to a reduction in excitation efficiency of the phosphor, and the latter (i.e., the other lighting device described above) will be subjected to a reduction in transparency of the mold member. Thus, none of the lighting devices can obtain a high-brightness illuminating light over the long term.